I cant tell if you are saying only model that gets the update is black, or if you mean that only black ones are permanently usable outdoors? If the latter is the case, then i'm happy to tell you the Ultramarine Battle Tank was added in Cata as a rare drop from Archaeology. The mount is learned from an item called The Scepter of Azj'Aqir, which has a low drop rate from the Tol'vir archaeology branch which has digsites in Uldum only. It's pretty much a permanent Blue Qiraji Battle Tank that can be used anywhere, just like the Black Qiraji Battle Tank.
http://www.wowhead.com/item=64883/scepter-of-azjaqir
Ofc, you wouldn't get a title or any kind of prestige and recognition as if you were a scarab lord, but it's still a must grab for anyone that wishes they could ride these chitinous critters outside of the raid.
Also If you mean that first thing I said, then I guess i stand corrected about the other colors getting an HD model
. But i'm sure it would happen eventually!